1. Field of the Invention
The present invention relates to differential signal generator circuits in which a first amplifier compares the input signal with a threshold voltage and outputs differential signals and a second amplifier adjusts the threshold voltage in response to the differential signals. More particularly, the invention relates to differential signal generator circuits that allow for easy adjustment of the cross-point of the differential signals.
2. Background Art
In a typical optical signal receiver, etc., the optical signal is converted into an electrical signal by a photodiode preamplifier and then into differential signals by a differential signal generator circuit which are then discriminated by a discriminator circuit. The optical signal is a two-state signal that has a high intensity state (called the “mark” or “mark state”) and a low intensity state (called the “space” or “space state”), each representing a different value. It should be noted that generally the optical signal includes greater noise when in the mark state than when in the space state. Therefore, it might happen that noise in the optical signal in the mark state prevents the discriminator circuit from properly discriminating the differential signals. To avoid this, the cross-point of the differential signals input to the discriminator circuit may be adjusted such that the discrimination level is slightly lower than the average value of the input signal. In many cases the appropriate amount of such adjustment of the cross-point is 10 mV or less, although this may vary depending on the performance of the amplifier.
One well known differential signal generator circuit includes a first amplifier for comparing the input signal with a threshold voltage and outputting differential signals and a second amplifier for adjusting the threshold voltage in response to the differential signals (see, e.g., JP-A-7-250105).
FIG. 7 is a block diagram showing a conventional differential signal generator circuit. Referring to FIG. 7, a first amplifier 11 compares the input signal VIN with a threshold voltage Vth and outputs differential signals V+ and V−. A second amplifier 12 made up of a differential amplifier receives the differential signals V+ and V− and outputs an output voltage Vout. One end of a resistance 13 is connected to the output of the second amplifier 12 to convert the output voltage Vout to the threshold voltage Vth. That is, in response to the differential signals V+ and V−, the second amplifier 12 automatically adjusts the threshold voltage Vth to be equal to the average value (or voltage) of the input signal VIN. The other end of the resistance 13 is grounded through a capacitance 14. The discriminator circuit (15) is a clock and data recovery (or CDR) circuit, etc., and determines, based on the average values of the differential signals V+ and V− output from the first amplifier 11, whether the input signal VIN is at a logical high level or a logical low level.
This conventional differential signal generator circuit is designed such that if the optical signal includes significant noise when in its mark state, a voltage may be externally applied through an adjustment terminal ADJUST and a resistance 17 to adjust the threshold voltage Vth to be slightly lower than the average value (or voltage) of the input signal VIN.